I'm pretty sure they're pronouncing 你 as nie, and I've never heard it pronounced that way before. Looking through the Wiktionary page for 你, I don't see any dialects, Chengdu included, that pronounce it as anything like nie.
In their other song 你，我 they're pronouncing 你 as ni as I'd expect, though it might be a different singer (can't really tell).
Is this a pronunciation that other people actually use? Or are they just doing it for the musical effect?